Microstrip frequency-scan antenna

ABSTRACT

An antenna comprises a microstrip transmission line which includes a strip conductor and a ground plane conductor separated by a dielectric substrate; a portion of the substrate being adapted to enable energy within the substrate to radiate away from the microstrip transmission line. To permit frequency-scanning, the substrate has an antenna portion formed therein which is adapted to permit RF energy supplied to the microstrip transmission line to be directionally radiated away from the microstrip transmission line at the antenna portion, the direction of radiation being a function of the frequency of the supplied energy. The substrate may have greater capacitance at said antenna portion than at other portions of said substrate.

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured, used, and licensedby or for the U.S. Government for governmental purposes without thepayment to the inventors of any royalty.

CROSS-REFERENCE TO RELATED APPLICATION

This application is related to Ser. No. 07/833,259, filed on Feb. 10,1992 by the same inventors, and titled SWITCHABLE SCAN ANTENNA ARRAY,the disclosure of which are incorporated by reference herein, now U.S.Pat. No. 5,144,320.

BACKGROUND OF THE INVENTION

The invention relates generally to a frequency-scan antenna for themillimeter-wave region, and more particularly to such an antenna havinga ramped-microstrip construction.

Frequency-scan antennas are employed to provide inertialess electronicscan capability in the millimeter-wave region, particularly for radarsystems. The inertialess scan feature is particularly important forsurveillance, obstacle-avoidance and target-acquisition radars.

Antennas and antenna arrays of background interest are described inStern et al., A MM-Wave Homogeneous Ferrite Phase Scan Antenna,Microwave Journal, Vol. 30, No. 4, pp. 101-108 (April 1987); Borowick etal., Inertialess Scan Antenna Techniques for Millimeter Waves, 9thDARPA/Tri-Service MMWave Conference Record (1981); and Collier,Microstrip Antenna Array for 12 GHz TV, Microwave Journal, Vol. 20, No.9, pp. 67-71. See also Stern et al., U.S. Pat. No. 4,754,237 issued Jun.28, 1988, titled Switchable Millimeter Wave Microstrip Circulator. Thecontents of all noted prior art materials are incorporated by referenceherein.

SUMMARY OF THE INVENTION

Despite the advantages of the known systems, there remains a continuingneed for a planar design for an electronic-scan antenna which is simple,efficient and cost-effective. The present invention satisfies this needby providing a frequency-scan antenna which a microstrip-typetransmission-line structure, mountable on a single microstrip substrate,offering the advantages of a small planar footprint, simpleconstruction, light weight, and low loss.

According to a particularly advantageous embodiment of the invention, anantenna comprises a microstrip transmission line which includes a stripconductor and a ground plane separated by a dielectric substrate; aportion of the substrate being adapted to enable energy within thesubstrate to radiate away from the microstrip transmission line. Topermit frequency-scanning, the substrate has an antenna portion formedtherein which is adapted to permit energy supplied to the microstriptransmission line to be directionally radiated away from the microstriptransmission line at the antenna portion, the direction of radiationbeing a function of the frequency of the supplied energy. The antennaportion may have greater capacitance than other portions of thesubstrate.

Other features and advantages of the present invention will becomeapparent from the following description of the invention which refers tothe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified perspective view of an antenna according to apreferred embodiment of the invention.

FIG. 2 is a plan view of the antenna of FIG. 1, the conductor strip notbeing shown, schematically showing its radiation pattern.

DESCRIPTION OF A PREFERRED EMBODIMENT

FIG. 1 is a simplified perspective view of an antenna according to anembodiment of the invention. A basic microstrip structure 10 is formedby a conductor 12, a dielectric substrate 14 and a ground planeconductor 16. The dielectric substrate 14 is typically about 0.010-inch(0.254-mm) thick Duroid (trademark), which is a composition of Teflon(trademark) and fiberglass.

At a ramp portion 18 the thickness of the substrate is increased toabout 0.070-inch (1.778-mm) to form a ramp up to a dielectric platform20 about 0.070-inch thick. The conductor 12 runs up the ramp portion 18and continues to run across the platform 20.

Formed on one side of the platform 20 is a series of evenly spacedperiodic slots. According to one example of the invention, for operationat about 35 GHz, the slots are about 0.140-inch (3.556-mm) apart andeach slot is about 0.008-inch (0.2032-mm) wide and 0.008-inch deep. Thespacing is a function of the wavelength and the dielectric constant.

The RF signal injected into the microstrip line enters into the thickerhigh-dielectric-constant platform section with low transition andtransmission loss. The top microstrip conductor and bottom ground plane,together with the high-dielectric-constant platform section, confine theenergy to be radiated out of the side of the antenna. As the signaltravels through the antenna section and experiences a side wall slot, aportion of the energy is radiated out of the slot, the slot being adiscontinuity for the propagating wave. This occurs for each successiveslot, each slot radiating a portion of the incident power. The periodicnature of the energy radiating from the slots results in the formationof an antenna beam pattern. Any residual RF energy traveling down thelength of the antenna beyond the slotted section is dumped into anabsorbing load.

By changing the frequency of the input energy, the position of theradiating beam can be shifted as shown in the top view of FIG. 2, inwhich the solid line B₁ shows the radiation pattern at one frequency andthe dotted line B₁ ' shows the radiation pattern at another frequency.

Adjacent approximately the full length of the platform 20, the substratehas substantially the same width as the platform 20. Near the rampportion 18 the substrate has tapered portions 22 on each side where thesubstrate widens, so that away from the platform, the substrate extendsaway from the microstrip by approximately 1 to 2 times the width of themicrostrip. These widened portions of the substrate provide a widenedground plane 16, which helps to contain the electric field in thesubstrate between the conductor 12 and the ground plane 16.

On the other hand, the platform 20 is made of a low-loss microwave-typedielectric material, for example MgTi. Its dielectric constant is about12, while that of the Duroid substrate is about 2. With the highdielectric constant of the platform 20, the extended ground plane is notnecessary to contain the field and furthermore would distort theradiation pattern of the antenna if it were present.

The precise location of the tapered portions 22 is not believed to becritical. The ground plane should start to widen between the last slotand the top of the ramp, and preferably near the top of the ramp.

Although the present invention has been described in relation to aparticular embodiment thereof, variations and modifications and otheruses will become apparent to those skilled in the art. It is preferred,therefore, that the present invention be limited not by the specificdisclosure herein, but only by the appended claims.

What is claimed is:
 1. An antenna, comprising:a microstrip transmissionline which includes a strip conductor and a ground plane conductorseparated by a dielectric substrate; the substrate having means enablingenergy within said substrate to radiate away from said microstriptransmission line.
 2. A frequency-scan antenna, comprising:a microstriptransmission line which includes a strip conductor and a ground planeconductor separated by a dielectric substrate; the substrate having anantenna portion formed therein having means permitting RF energysupplied to said microstrip transmission line to be directionallyradiated away from said microstrip transmission line at said antennaportion, the direction of radiation being a function of the frequency ofsaid supplied energy.
 3. An antenna as in claim 2, wherein said antennaportion is defined by a platform between said strip conductor and saidground plane conductor at said antenna portion which is substantiallythicker than at other portions of said substrate.
 4. An antenna as inclaim 3, wherein said means comprises lateral slots formed in saidplatform permitting lateral radiation therefrom of said supplied energy.5. An antenna as in claim 4, wherein said slots are spaced withsubstantially equal spacing, thereby permitting the radiated energy tobe scanned by changing the frequency of the supplied energy.
 6. Anantenna as in claim 5, wherein said spacing is selected as a function ofthe frequency of the supplied energy and of the dielectric constant ofthe platform.
 7. An antenna as in claim 2, wherein said substrate hasgreater capacitance at said antenna portion than at said other portionsof said substrate.
 8. An antenna as in claim 2, wherein the substratehas a higher dielectric constant at said antenna portion than at saidother portions.
 9. An antenna as in claim 8, wherein the dielectricconstant at said antenna portion is substantially 6 times that at saidother portions.
 10. An antenna as in claim 9, wherein the dielectricconstant at said antenna portion is substantially 12 and that at saidother portions is substantially
 2. 11. An antenna as in claim 8, whereinsaid antenna portion is defined by a platform between said stripconductor and said ground plane conductor at said antenna portion whichis substantially thicker than at other portions of said substrate. 12.An antenna as in claim 11, wherein said means comprises lateral slotsformed in said platform permitting lateral radiation therefrom of saidsupplied energy.